1. Field of the Invention
The subject invention relates to a system and method for determining the absolute and relative position, velocity and attitude of a vehicle, such as a spacecraft or a terrestrial vehicle, utilizing modulated radiation emitted by various celestial sources. In particular, the present invention directs itself to a mobile receiver for detecting signals generated by celestial sources of modulated radiation. The mobile receiver may be mounted on a spacecraft, satellite, planetary rover, or other vehicle, and the received signal is used to calculate navigational data for the vehicle. More particularly, this invention directs itself to a clock or timer in communication with the mobile receiver for generating a timing signal corresponding to the reception and detection of the modulated celestial radiation. The timing signal is used to calculate a time difference between predicted and measured reception of the timing signal at the mobile receiver. Incorporating predetermined models of the modulated celestial radiation arrival times within an inertial reference frame, navigational data, such as a position offset of the mobile receiver, is then be determined from the time difference.
Further, the mobile receiver is in communication with a digital memory system. The digital memory system stores information including the positions and pulse timing model parameters of known sources of modulated celestial radiation with respect to the chosen inertial reference frame. Further, this invention directs itself to a navigational system including a processing means for calculating navigational data for spacecraft, satellite, planetary rover, or vehicle based upon the calculated time difference in combination with the known positional and timing model data of the sources of celestial radiation stored within the digital memory system.
Additionally, the invention directs itself to a navigational system and method allowing for calculation of either an absolute position of the spacecraft with regard to an absolute reference, or a relative position being calculated with respect to a relative selected position, such as another spacecraft. Further, the processing means allows for the calculation of error correction data which may be applied to the computed navigational data. Errors due to relativistic effects are also computed and corrected for.
2. Description of the Prior Art
Celestial objects, or sources, have been utilized throughout history as navigational aids. The motions of the Sun, Moon, and planets, as observed from the Earth's surface, have provided the concept of time, and are a form of a celestial clock. Using catalogued almanac information, the observation of visible stars have provided travelers on the Earth a means to determine position information relative to observation stations fixed on the Earth. As these methods have matured over time, and with the addition of instrumented time clocks, or chronometers, the performance of navigation methods has improved.
Navigation of vehicles above and beyond the Earth's surface have benefited from this knowledge of celestial source-based navigation. Satellites orbiting the Earth and spacecraft traveling throughout the Solar System have relied on celestial sources to successfully complete their missions. Additionally, celestial source navigation systems have been augmented with human-made systems to further increase vehicle and spacecraft navigation performance. In fact, a wide variety of methods have been used to compute the navigation information of spacecraft which have traveled around the Earth, through the Solar System, and beyond the Solar System's outer planets, as far as the heliopause.
Navigation of spacecraft is defined as determining the spacecraft's three-dimensional position, velocity, and attitude at a specific time or times. Position determination of near-Earth missions has included the use of ground-based radar and optical tracking, Earth-limb horizon sensors, Sun sensors, and Global Positioning System (GPS) receivers. These sensors use knowledge and observations of celestial objects and phenomena to determine position relative to the Earth. The GPS system produces signals from multiple transmitting satellites that allow a receiver to determine its position from the ranges to the transmitting satellites. Position determination of spacecraft on interplanetary missions has utilized Doppler radar range measurements, standard radio telemetry, and target body imagery. Attitude, or orientation, of spacecraft has typically been determined using magnetometers, gyroscopes, star cameras, star trackers, Earth-limb horizon sensors and Sun sensors. Time has often been determined using an on-board clock on the spacecraft, or through periodic computer updates from ground control stations.
Radar range and tracking systems have been the predominant system for tracking and maintaining continuous orbit information of spacecraft. In order to compute position of spacecraft, radar range systems compute the range, range-rate, and/or the angular orientation angles to the spacecraft relative to the radial direction from a radar tracking station to the spacecraft. This is achieved primarily through the reflection of signals transmitted from an Earth observing station by the space vehicle structure and measurement of the transmitted signal round-trip time. Although this system requires no active hardware on the spacecraft itself, it does require an extensive ground tracking system and careful analysis of the measured data against an electromagnetically noisy background environment. Using the best known position of the observing station and processing multiple radar measurements, the vehicle's orbit parameters can be computed. The position of the vehicle can be propagated analytically ahead in time using standard orbital mechanics along with known models of Solar System object's gravitational potential field; as well as any known disturbance or perturbations effects, such as object body atmospheric drag. This propagated navigation solution is then compared to subsequent radar measurements and the navigation solution is collected for any determined errors. This process continues until a navigation solution converges to within the expedition's required parameters. However, vehicle maneuvers or any unaccounted-for disturbances will affect the trajectory of the vehicle. Without exact knowledge of these maneuver dynamics or disturbance effects, it is necessary for the propagation and radar measurement comparison to continue throughout the flight.
As the spacecraft moves further away from Earth observation stations, the error in navigational data increases. To achieve the necessary range determination, the radar system requires knowledge of the observation station's position on the Earth to great accuracy. With sophisticated surveying systems, including GPS, such accuracy may be achieved. However, even with this precise knowledge, the position measurement can only be accurate to a finite angular accuracy. The transmitted radar beam, along with the reflected signal, travels in a cone of uncertainty. This uncertainty degrades the position knowledge in the transverse direction of the vehicle as a linear function of distance. As the vehicle gets more distant, any fixed angular uncertainty reduces the knowledge of vehicle position, especially in the two transverse axes relative to the range direction. These axes are along-track of the vehicle's velocity and cross-track, or perpendicular, to the vehicle's velocity and radial direction. Even utilizing interferometry, using the difference between multiple signals compared at two ranging stations, the angular uncertainty rapidly grows above acceptable limits.
Alternatively, many deep space spacecraft have employed active transmitters to be used for navigational purposes. The radial velocity is measured at a receiving station by measuring the Doppler shift in the frequency of the transmitted signal. The spacecraft essentially receives a “ping” from the Earth observation station and re-transmits the signal back to the Earth. Although improvements in the radial direction range and range-rate measurements are made utilizing such system, transverse axes, including along-track and cross-track axes, errors still exist, and this method has errors that also grow with distance.
Optical tracking measurements for spacecraft position and orbit determination are completed in a similar fashion as radar tracking. Optical tracking uses the visible light reflected off a vehicle to determine its location. Some optical measurements require a photograph to be taken and the vehicle's position is calculated after analysis of the photograph and comparison to a fixed star background. Real-time measurements using such systems are typically not easily achieved. Additionally, optical measurements are limited by favorable weather and environmental conditions. Because most missions have been oriented around planetary observation, augmentation to the ranging navigation system can be made within the vicinity of the investigated planet. By taking video images of the planet and comparing to known planetary parameters (such as diameter and position), the video images can determine position of the spacecraft relative to the planet. Since, often the objective is to orbit the planet, only relative positioning is needed for the final phase of the flight and not absolute position.
To aid the position determination process, an accurate clock is a fundamental component to most spacecraft navigational systems. On-board clocks provide a reference for the vehicle to use as its own process timer and for comparison to other time systems. Atomic clocks provide high accuracy references and are typically accurate to within one part in 109-1015 over a day. As calculated by Melbourne in “Navigation Between the Planets”, Scientific American, Vol. 234, No. 6, June 1976, pp. 58-74, in order to track radio signals at accuracies of a few tenths of a meter, a clock with nanosecond accuracy over several hours is needed. This requires the clock to be stable within one part in 1013. As early chronometers helped improve navigation over the Earth's ocean, more accurate chronometers will help navigation through the solar system.
To gain increased autonomy in spacecraft operation, it is desirable to develop systems other than the operationally intensive human-controlled radar and optical position determination methods. Methods that employ celestial sources, which provide positioning capabilities and do not require labor-intensive operations, remain attractive.
In order to navigate using celestial objects, precise knowledge of their positions relative to a defined reference frame at a selected time epoch is required. Catalogued ephemeris information of Solar System objects provides this reference frame information. The Sun, Moon, and planets all translate within the Solar System in a reference frame viewed from the Earth. Since the orbits of the objects are near-Keplerian, and nearly circular, this translational motion is nearly periodic, repeating after a certain time span elapsed. It is exactly this periodicity that leads to the concept of time. The motion of these objects, or their angular displacement in their orbit, can be interpreted as a clock measuring time.
Although seemingly “fixed” with respect to a frame on the Earth, the visible spectrum stars can also provide a measurement of time and, therefore, can be interpreted as a navigational reference. In this case, however, it is the Earth or spacecraft that provides the time measurement, by rotating or translating with respect to the fixed background of stars. The extremely large distances to the stars in the Milky Way galaxy and other galaxies essentially create the illusion that the stars are fixed. Just as the Solar System rotates, however, so does the Milky Way rotate and the Galaxy translates with respect to neighboring galaxies. Thus, objects are continually speeding away and towards the Earth at all times. However, the motion of the stars is so slow compared to many other measurements of time that this motion is perceived as fixed.
In addition to providing a measure of time, just as humans “triangulate” their Earth position relative to identifiable landmarks, it is conceivable to use persistent star light as markers for triangulating position. Observing known stars allows a spacecraft to initially estimate its orientation and begin a process of determining its position relative to another object. However, due to the large number of visible stars, detecting specific stars can be time-consuming due to necessary almanac database searches. Also, since there is no method of determining “when” the visible light was sent from the stars and because the stars are located so far away, determining range information from an individual star to help “triangulate” a spacecraft's position is problematic. Only during the instance of occultation, or when a known celestial body passes in front of a selected star while it is being viewed, allows a dependable method of position determination directly from star light.
However, individual stars that do have a uniquely identifiable signal, for example, whose signals are periodic, can be utilized directly as celestial sources for navigation purposes.
Astronomical observations have revealed several classes of celestial objects that produce unique signals. A particularly unique and stable signal is generated by pulsars. It is theorized that pulsars are rotating neutron stars, which are formed when a class of stars collapse. Conservation of angular momentum asserts that, as the stars become smaller, or more compact, they rotate faster. Neutron stars rotate with periods ranging from 1.5 ms to thousands of seconds. A unique aspect of this rotation is that for certain classes of pulsars, the rotation can be extremely stable, providing thereby a stable source of radiation modulated by the neutron star's rotation. The most stable pulsars have stabilities on the order of 10−14 when measured on time scales of a year, which is comparable to the best terrestrial atomic clocks. No two neutron stars have been formed in exactly the same manner, thus their periodic signatures are unique. Because pulsars provide signals that are unique, periodic and extremely stable, they can assist in navigation by providing a method to triangulate position from their signals. Pulsars can be observed in the radio, optical, X-ray, and gamma-ray ranges of the electromagnetic spectrum. They exist as isolated neutron stars or components of multiple star systems. Several classes of high and low mass binary companions exist for pulsar stars. The cyclic eclipsing of a pulsar by its binary companion provides a secondary periodic signal from these sources.
Downs, in “Interplanetary Navigation Using Pulsating Radio Sources”, NASA Technical Reports, N74-34150, Oct. 1, 1974, pp. 1-12, presented a method of navigation for orbiting spacecraft based upon radio signals from a pulsar. However, both the radio and optical signatures from pulsars have limitations that reduce their effectiveness for spacecraft missions. In order to be effective, optical pulsar-based navigational systems would require a large aperture to collect sufficient photons, since few pulsars exhibit bright optical pulsations. To date there are only five pulsars that have been observed to emit pulsed visible radiation. This small number of sources severely limits the options for an optical-based variable source navigation system. The large number of fixed radiation visible sources requires precise pointing and significant processing to detect pulsars in the presence of bright neighboring objects. This is not attractive for vehicle design. At the radio frequencies which pulsars emit (˜100 MHz-few GHz) and with their faint emission, large antennae (likely 25 m in diameter or larger) would be required, which would be impractical for most spacecraft. Also, neighboring celestial objects, including the Sun, Moon, close stars, and Jupiter, as well as distant objects, such as supernova remnants, radio galaxies, quasars, and the galactic diffuse emissions are broadband radio sources that could obscure the weak pulsar radio signals, as shown by Wallace in “Radio Stars, What They Are and The Prospects for Their Use in Navigational System”, Journal of Navigation, Vol. 41, September 1988, pp. 358-374. The low radio signal intensity from pulsars would require long signal integration times for an acceptable signal to noise ratio.
Chester and Butman in “Navigation Using X-Ray Pulsars”, NASA Technical Reports, N81-27129, Jun. 15, 1981, pp. 22-25, propose the use of pulsars emitting in the X-ray band as a better option for navigation. Antennae on the order of 0.1 m2 could be used for X-ray detection, which is much more reasonable than a large radio antenna. Additionally, there are fewer X-ray sources to contend with and many have unique signatures, which do not get obscured by closer celestial objects. One complicating factor is that many X-ray sources are transient in nature. The transient sources are only detectable at irregular intervals as a result of a modulation in the accretion rate onto the celestial source. Typically, the “steady” sources (such as many of the rotation-powered pulsars) would be used for navigation. By cataloging pulsar positions and recording their signal periodicity and identifying parameters, as well as the data from other types of modulated celestial radiation sources, a table of candidate stars can be created for use in navigation. These catalogs can then be stored in data memory format for use by algorithm processes onboard vehicles that detect modulated radiation signals. Maintenance of these catalogs, and timely dissemination of data updates, is required for a high performance navigation system.